In the past different suspension solutions for motorcycles have been developed that are provided with a single longitudinal arm, of the “push” or “pull” type, and with a damper with coaxial spring. In most of such so-called “single-arm” solutions, at least as regards applications on scooters or on tilting vehicles in general, the steering system is made by simply constraining the support pipe of the suspension arm to the frame, using bearings and/or fifth wheels.
A typical example of application of the pushing arm system is found on the “Vespa” scooter, whereas the pulling arm system is mounted for example on the “MP3” tilting three-wheeled scooter produced by Piaggio. The pushing arm system is made up of a single rigid arm, on the lower end of which a small arm is articulated, ending on the pin of the front wheel. Such a small arm has the lower end of the suspension fixed onto it, which in turn fixes on top also to the rigid arm. The assembly thus takes up the shape of a parallelogram. The pulling arm system constitutes a variant of the pushing arm system.
The characteristics and the advantages of this kind of single-arm suspension for motorcycles can be summarised as follows:
possibility of quickly replacing the wheel, since the connection of the wheel itself to the hub is easy to access;
smooth running, ensured by the movement of the support aim of the wheel on bearings and by the absence of transversal loads on the damper;
possibility of obtaining different behaviour of the suspension during braking, according to the positioning of the centre of the wheel as regards the pivot of the arm and the type of attachment of the brake clip to the suspension.
However, the aforementioned advantages are counterbalanced by a series of defects that make single-arm suspensions unsuitable for some specific applications. The requirements of rigidity and low bulk, indeed, significantly limit the length of the arm of the suspension in relation to a correct operation of the system. Consequently, the stroke available to absorb the unevenness of the road is equally limited. For the same reasons it is complex to use wheels with a large diameter associated with such a type of suspension.
The circular trajectory of the suspension arm and the type of steering system (steering axle fixed as regards the frame) also mean that the “caster trail” of the wheel as regards the steering axle itself varies depending on the stroke of the suspension and cannot be controlled precisely. As known, by caster trail, or “front wheel trail”, of a motorcycle we mean the distance between the projection to the ground of the perpendicular passing through the pin of the front wheel or, in other words, the point of contact of the front wheel and the ground, and the point of intersection of the rotation axis of the steering again with the ground. The aforementioned variation of the caster trail can, in some cases, cause sudden reactions of the steering for long strokes of the suspension, as occurs for example in braking manoeuvres.
Moreover, the large number of components of the single-arm system complicates the assembly steps of the assembly and involves higher costs than a conventional fork suspension.
In the case of use of a single-arm suspension on the front end of tilting vehicles in general, like for example the “MP3” tilting three-wheeled scooter produced by Piaggio, it is finally necessary to foresee connection elements to the frame having high rigidity values, as well as suitable linkages for all driving conditions, capable for example of carrying out the asymmetric blocking of the wheels during braking. The suspension system must therefore ensure high precision and ease of assembly, as well as suitably limiting the excursions of the components depending on the spaces available following complex combined rolling and steering movements.
Therefore, suspension systems for motorcycles have been made provided with a double arm cantilever-mounted as regards the wheel, like for example those described in documents U.S. Pat. No. 2,953,395 and DE 808191 C. Such suspension systems, however, do not appear to be able to be advantageously applied on a tilting three or four-wheeled motorcycle. Document WO 2005/051754 A1, on the other hand, describes a suspension system for motorcycles provided with a double arm mounted on each side of the wheel. Finally, document FR 2940242 A1 describes a classical single-arm suspension system for a motorcycle.